A pressure loss portion is arranged in an intake air passage or an exhaust gas passage of the engine, which is an example of a fluid passage. The pressure loss portion is arranged to cool intake air or purify exhaust gas while causing pressure loss by friction with fluid. An intercooler is one of such pressure loss portions and is arranged in a portion of the intake air passage that is located downstream of a supercharger, which supercharges intake air. The intercooler cools intake air of which the temperature has been increased by being pressurized.
If an anomaly such as clogging occurs in the pressure loss portion, the original function declines. Thus, devices and methods that detect an anomaly of the pressure loss portion have been proposed. As an example, a device is proposed that estimates an anomaly of a turbocharger and an anomaly of an intercooler from their temperatures (for example, refer to Patent Document 1). A sensor of the device detects a discharged air temperature, which is the temperature of a portion that is located upstream of the intercooler and between the turbocharger and the intercooler, and a temperature of a portion that is located downstream of the intercooler. Furthermore, the device calculates cooling efficiency from these temperatures and compares the calculated cooling efficiency with a map showing the cooling efficiency of the intercooler in a normal state. When the cooling efficiency declines to a value less than the normal value, the device determines that the intercooler is anomalous.